1. Field of the Invention
The invention concerns a device for the cooling of hot gaseous suspensions of solids, particularly hot gaseous TiO.sub.2 suspensions resulting from the production of titanium dioxide by oxidation of titanium tetrachloride in the vapor phase.
In many chemical processes for the production of solids, the reactants are reacted in the vapor phase at high temperatures with the result that the desired solids are usually obtained finely divided and suspended in the gas stream.
This involves the problem of cooling these hot gaseous suspensions in suitable devices (heat exchangers) so that the solid and the gaseous components can subsequently be separated in suitable separators like cyclones, bag filters, electric precipitators etc.
Examples of such reactions are the production of metal or metalloid oxides like TiO.sub.2, Fe.sub.2 O.sub.3 and SiO.sub.2 by oxidation of their halogenides in the vapor phase and the production of carbon black.
Particularly the production of titanium dioxide by vapor phase oxidation of titanium tetrachloride with oxygen or an oxygen-containing gas like air has in recent times gained in importance over the older production methods in which titaniferous ores are digested with an acid since, in vapor phase oxidation, only solid and gaseous end products are obtained, thus eliminating the problems connected with the disposal of the waste acid.
The titanium tetrachloride is oxidized in a reaction chamber, usually with oxygen used as the oxidant. For the purpose of enhancing certain properties of the pigment, small quantities of other substances like water, aluminum chloride and/or silicon tetrachloride and/or zirconium tetrachloride may be added during oxidation.
Oxidation of titanium tetrachloride is a weakly exothermic reaction. The feed materials are therefore preheated, e.g. by indirect heating method, so as to sustain the reaction. Instead of or in addition to preheating, the energy may also be added directly in the form of an auxiliary flame that is, for instance, obtained by burning carbon monoxide.
The oxidizing agent, i.e. oxygen or air, is added at an excess relative to the stoichiometric quantity of oxygen required for the oxidation of the titanium tetrachloride and any other oxidizable substances present. The solids obtained in the reaction mixture are finely distributed titanium dioxide and, as the case may be, admixtures of small quantities of other oxides resulting from the cooxidation of additives. The gaseous constituents of the reaction mixture are chlorine, oxygen, and nitrogen, and, as the case may be, carbon dioxide and low quantities of steam or hydrochloric acid formed by the reaction of the steam with chlorine.
The most important constituent of the reaction mixture is the titanium dioxide obtained in the oxidation of the titanium tetrachloride, which is present very finely distributed in the reaction gases as a fumelike gaseous TiO.sub.2 suspension. The TiO.sub.2 particles thus obtained are sticky in the temperature range between discharge from the reactor down to the range of about 350.degree. to 700.degree. C. and therefore show a strong tendency to clinging to surfaces in the form of tough, tightly adhering deposits which show little or no tendency to come off on their own. Silicon dioxide and carbon black behave in a similar manner. It so happens that deposits growing thicker and thicker form in the tubes, thus reducing the effective inner diameter of the tubes and, as a consequence, impairing heat transfer and eventually requiring operations to be suspended.
In the hot reaction zone, the primary particles tend to grow.
When retained too long in the hot zone, the TiO.sub.2 particles produced will aggregate, forming irregularly shaped pigment particles with widened particle size range and deteriorated pigmentary properties, e.g., reduced lightening and hiding power, or impaired dispersibility as a consequence of partial sintering of the particles.
The TiO.sub.2 formed in the reaction zone must, for this reason, be cooled down rapidly as soon as it leaves the reaction chamber.
To obtain optimum pigment quality, the gaseous TiO.sub.2 suspension must be cooled down within a certain limited distance (cooling duct) and within a certain limited period of time and to such a degree that thereafter TiO.sub.2 can be selected from the gaseous constituents in a manner as customary in normal practice. A device (heat exchanger) suited for this purpose must be made of a material that is resistant to hot chlorine gas, it must moreover not allow deposits of fine-sized TiO.sub.2 to form on its walls during operation, and any thin deposits forming after all must be easily removable, e.g., by the introduction of chemically inert scrub solids without exposing the interior walls of the device to significant damages by abrasion. Only if these conditions are met, will consistent heat exchange between the hot reaction mixture and the cooling device be obtained, which is an essential requirement for the production of titanium dioxide of consistent quality in the optimum particle size range.
2. Description of the Prior Art
According to U.S. Pat. No. 2,721,626, the hot gaseous TiO.sub.2 suspension is cooled in an aluminum tube consisting of a series of 180.degree. bends. The formation of deposits on the walls is prevented by adding cold, chemically inert solid particles like sand or calcined TiO.sub.2 larger in particle size than the TiO.sub.2 present in the reaction mixture. Although the scrubbing effect of the solid particles is very good in the 180.degree. bends, abrasion is at the same time high.
According to U.S. Pat. No. 2,899,278, it is proposed to use a titanium dioxide milled in a fluid energy mill, then washed with water and finally granulated by heat treatment as scrub solids material. Although this material shows good scrubbing effect and although lower amounts may be used than in the case of coarser materials, this does not solve the problem of abrasion in the bends of the heat exchanger either.
According to German Patent Application No. DE-OS 17 67 798, the cooling tube is, in its essential parts, to be made of magnesium or a magnesium compound on which the TiO.sub.2 made by vapor phase oxidation shows less tendency to depositing than on aluminum. The problem of abrasion in the bends of the cooling tubes does, however, continue to exist.
In German Patent Application No. DE-OS 19 23 614, water-soluble salts, free from water, e.g., salts of alkali and earth alkali metals, particularly sodium chloride, are proposed to be used as scrub solids; they must, however, be washed out in the further course of the pigment production process as they are foreign to the system and do not solve the abrasion problem either.
Also other ways have been proposed:
In the German Patent Application No. DE-OS 20 05 011, it is proposed to mix cold gas into the hot gaseous solids suspension and with a change of direction to introduce it into an expanding chamber, in which the formation of deposits is prevented mechanically, whereafter the product stream is once more subjected to a change of direction. The mechanical installations complicate the device and increase its susceptibility to abrasion.
Finally, German Pat. No. DE-PS 12 83 818 proposes to introduce the hot gaseous TiO.sub.2 suspension into a cooling chamber, which conically tapers off at its lower end, by introducing a cooling gas into the cooling chamber in counterflow to the hot reaction mixture, in which process the cooling gas must be produced by a complicated method of cleaning the reaction gases.